Regarding electric power supply, various proposals are presented from a viewpoint of environmental load reduction. For instance, Patent Document 1 discloses a photovoltaic power generation system constituted of solar cells, a storage battery, and an interconnection inverter having a self sustaining function, which is operated as an interconnection photovoltaic power generation system when the system power supply is normal, and in a disaster situation the inverter is operated in a self sustaining mode using the solar cells and the storage battery as power sources so as to supply electric power to a specific load. In this photovoltaic power generation system, the inverter has a bidirectional function, and supplemental charging of the storage battery is performed with the system side electric power by operating the inverter in a converter mode when the system power supply is normal. It is also proposed that the supplemental charging of the storage battery should be performed after the interconnection photovoltaic power generation operation is stopped, and further that the supplemental charging of the storage battery should be performed during a time slot to which a midnight power rate is applied. In addition, Patent Document 2 proposes an electric power selling system by the photovoltaic power generation.